1. Field of the Invention
The present invention relates to a power supply module adapted to power a control circuit of a switching mode power supply. More specifically, the present invention relates to a power supply module adapted to increase the operation efficiency of a switching mode power supply and to decrease the power loss incurred by the control circuit of the switching mode power supply when working in a light load or an open load status.
2. Descriptions of the Related Art
Over recent years, the requirements of the operation efficiency in a switching mode power supply have switched from a focus on the heavy or full load status to both the light and full load status. For example, a well-established American corporation, Intel Corp., has established requirements on the operation efficiency of a power supply so that when the power supply operates with a 5% load, the operation efficiency thereof should be no less than 80%.
When operating under a light load condition, the switching mode power supply is usually controlled to operate in the burst mode to decrease the power loss of the main circuit. And an analysis on the power loss under a light load has revealed that the control circuit loss accounts for a significant portion of the total power loss. Accordingly, it is possible to increase the efficiency of the switching mode power supply in the light load status by decreasing the power loss of control circuit, which may in turn be accomplished by lowering the supply voltage of the control circuit.
FIG. 1 illustrates a conventional switching mode power supply 1. The switching mode power supply 1 comprises a main circuit and a control circuit. For example, the main circuit further comprises an electromagnetic interference (EMI) filter 101, a bridge rectifier 103, a power factor correction circuit 105, an inverting circuit 109, a main transformer 111, a rectifier 115 and an output filter 119. The control circuit further comprises a power factor correction controller 107, a controller chip 117, a feedback circuit 121 and a protection circuit (not shown). The inverting circuit 109 may be a half-bridge structure, a full-bridge structure, and so on. The operations and functions of these elements are well-known by those skilled in the art, and therefore will not be described again.
Now, only the structure of the switching mode power supply 1 will be described. The power supply module of the switching mode power supply 1 typically supplies power to the control circuit of power supply 1. In the switching mode power supply 1, the power supply module includes a bias winding 113 coupled to the main transformer 111 and a filter module (not shown) which is coupled to the bias winding 113 and filters the voltage generated by the bias winding 113 and outputs a DC voltage to power the control circuit.
To increase the efficiency of the main circuit in this structure at rated power output condition, the secondary winding of the main transformer 111 is always designed to have a small number of turns. For instance, for a circuit with a rated output voltage of 19.5 V, if the secondary winding is designed to be three turns, then there will be three possibilities for the number of turns of the bias winding 113: one turn, two turns, or three turns. If the bias winding 113 has one turn, the output voltage generated by the bias winding would be only 6.5 V, regardless of the cross-regulation ratio. Thus the controller chip 117 would fail to work because its operating voltage threshold is always set to be at 10 V or higher. If the bias winding 113 has two turns, the output voltage would be 13 V without considering the cross-regulation ratio. Then there is no problem when operating at heavy load condition; however, when the circuit operates under light load condition, the output voltage will fall below the operating voltage range of the control circuit, which still causes control circuit unable to operate. Finally, if the bias winding 113 has three turns, the output voltage will always stay above 16 V, even under a light load. However, a relatively high voltage will be unfavorable for decreasing of the power loss incurred by the control circuit under a light load.
Accordingly, efforts still have to be made in the art to enable a power apparatus to operate normally with less power loss incurred by the control circuit under a light load or open load while securing a highly efficient power apparatus in a rated output status.